1. Field of the Invention
The present invention relates to an imaging lens for use on a digital imaging device for a monitor camera, a vehicular camera or the like having a solid-state imaging device, such as of CCD or MOS, and more particularly to a light-weighted small-sized imaging lens to be suitably applied for a cellular phone camera.
2. Description of Background Art
As the solid-state imaging device reduces in size and the number of pixels thereof increases, the imaging lenses for use on various digital imaging devices are required smaller in size and more favorable in optical performance obtainable. Particularly, for the imaging lenses to be mounted on the popular cellular phone, manufacture at lower cost is required in addition to that desire.
There is a proposal on an imaging lens that optical performances are favorably obtained by the provision of a lens with a diffractive optical surface while keeping the two or three lens structure (see JP-A-2004-191844 and JP-A-10-90596).
However, it is difficult for the two-lens structure, described in JP-A-2004-191844 and JP-A-10-90596, to satisfy the desire in respect of optical performances.
Meanwhile, the three-lens structure described in JP-A-10-90596 has a first lens, arranged closest to the object, having a negative refractive power, thus being made as a retro-focus type whose back focus is longer than the overall combined focal length. This results in an increased overall length, and thus the lens is not suited as a cellular-phone imaging lens where size reduction is desired.